Letter: Since when is child-rearing sole purpose of marriage, gay or not?

Question 6 has thankfully passed, but I would like to respond to the letter written by Lynda Kouroupis ("Allowing same-sex marriage destroys 'treasured institution'," Nov. 1. In her letter, Ms. Kouroupis states all the benefits of marriage as related to child rearing by a heterosexual couple only.

I'm wondering, should a heterosexual couple unable to have children (past child-bearing age, sterilized, etc.) be unable to marry? Should, after a certain number of years of attempting unsuccessfully to have children, such a marriage would no longer be considered valid?